In crane construction which comprises opposed pairs of end truck assemblies movable along a track and a long transverse support member between the end truck assemblies, it is common for skewing to occur wherein one end truck assembly gets ahead of or behind of the end truck assembly on the other side of the crane. This skew condition creates horizontal or lateral forces to be induced into the runway or track system on which the crane is operating. On most overhead or industrial cranes, the forces caused by this skewing action can be handled by the runway system or are not significant due to the short travel distances of the crane in the system. However, where cranes are utilized for maintenance platforms under a bridge to traverse a span of the bridge, the operation of the crane will be along a considerable length and a skew condition will increase and induce forces of damaging magnitude on the structure of the bridge.
In one type of maintenance platform that has been proposed, the platform utilized wound rotor motors which resulted in a runaway condition as the platform operated down the slope of the bridge.
In U.S. Pat. No. 4,505,207, an anti-skewing device for a bridge crane comprises a proximity switch near a first side of a first track and near a first end of the crane for detecting the proximity of the switch to the first side of the first track and thereby any skewing of the crane. There is an electrically activated brake for each set of wheels. A circuit connects the proximity switch to each of the brakes to slow one end of the crane relative to another end and thereby correct any skewing during movement of the crane.
Among the objectives of the present invention are to provide a crane with an anti-skewing device which will function to correct the skewing; which is relatively simple and inexpensive in construction; which requires minimum maintenance; and which is reliable.
In accordance with the invention; a crane with an anti-skewing device comprises a platform suspended under a bridge deck by two end truck assemblies supported by runways on a bridge. Each end truck assembly includes two sets of trolleys and an upper load bar laterally interconnects the two sets of trolleys. A lower load bar is suspended from the upper load bar by a kingpin for rotation about a vertical axis. Hinge tubes support the platform on the suspension assembly for pivotal movement in a direction lengthwise of the platform. Each end truck assembly is driven by a variable speed motor. As skewing occurs a mechanical sensing mechanism senses relative movement between the lower load bar and the upper load bar and provides a signal to an electronic circuit causing one or both of the motors associated with the end truck assembly on which the sensing mechanism is mounted to increase or decrease the speed of the motor and thereby correct the skew.